The fibroblast growth factor (FGF) family is composed of structurally related polypeptides that bind to 4 receptor tyrosine kinases (Fibroblast Growth Factor Receptors 1-4) and one kinase deficient receptor (Fibroblast Growth Factor Receptor 5) (Eswarakumar et al. (2005) Cytokine Growth Factor Rev 16:139; Ornitz et al. (2001) Genome Biol 2:3005.1; Sleeman et al. (2001) Gene 271:171). The FGF receptors can be activated by the FGF family, which includes 23 unique members to date (Eswarakumar et al. (2005) Cytokine Growth Factor Rev 16:139; Yamashita, T. (2005) Therapeutic Apheresis and Dialysis, 9:313). In contrast to Fibroblast Growth Factor Receptor 4, where only two splice variants are known, other family members such as Fibroblast Growth Factor Receptors 1-3 can be altered in their affinity for different FGFs by multiple splice variations (van Heumen et al. (1999) IUBMB Life 48:73).
Fibroblast growth factor receptors (FGFRs) are of great interest in cancer biology as these receptors regulate essential processes including cellular survival, motility, development, and angiogenesis. In humans, FGFRs have a highly conserved amino acid sequence across family members, including FGFR1, FGFR2, FGFR3, and FGFR4. These cell surface receptors include extracellular immunoglobulin-like domains, a transmembrane domain, and an intracellular tyrosine kinase (TK) domain. The interaction of FGFs with FGFR1-4 results in receptor homodimerization and autophosphorylation, recruitment of cytosolic adaptors such as FRS2 and initiation of multiple signaling pathways (Powers et al. (2000) Endocr Relat Cancer 7:165; Schlessinger, J. (2004) Science 306:1506).
FGFR4 is activated by FGF1, FGF2, FGF4, FGF6, FGF8 and FGF9 with decreasing efficiency, respectively (Ornitz et al. (1996) J. Biol. Chem. 271:15292). Although each of these FGFs also activates other FGFR family members, FGF19 is specific for FGFR4 (Xie et al. (1999) Cytokine JID-9005353 11:729). Activation of FGFR4 by FGFs requires binding of the ligand to heparin; although, FGFR4 can also be activated by heparin alone (Gao and Goldfarb, (1995) EMBO J. 14:2183). Many FGFs are broad-spectrum mitogens, whereas some induce cell motility, or alter the state of cellular differentiation (McKeehan et al. (1998) Prog Nucleic Acid Res Mol. Biol 59:135). In vivo, some FGFs have potent angiogenic properties, and others have been implicated in tissue remodeling, including wound repair (Wemer et al., (1994) Science 266:819).
Upon binding of a ligand to the extracellular domain of FGFR4, this receptor dimerizes, which results in the phosphorylation of residues in the TK domain. This phosphorylation induces the recruitment of signaling molecules to the intracellular domain, thereby activating one or more signaling pathways (Vainikka et al. (1992) EMBO J 11:4273; Vainikka et al. (1994) J. Biol. Chem. 269:18320). For example, FGFR4 associates with PLC-γ1, and an increase in MAP kinase activation and DNA synthesis upon FGF stimulation has been observed. Further interaction with other human FGFR family members may expand the signaling potential of FGFR4 and can provide not only signal diversification, but also signal amplification (McKeehan & Kan (1994) Mol Reprod Dev 39:69). An 85-kDa serine kinase has been found to negatively regulate tyrosine phosphorylation of FGFR4, but its exact function has not been elucidated (Vainikka et al. (1996) J Biol Chem 271:1270). Association of FGFR4 with NCAM has been demonstrated to mediate integrin-dependent adhesion (Cavallaro et al. (2001) Nat Cell Biol 3:650), which might play a decisive role in tumor metastasis.
FGFs and FGFRs play important roles in development and tissue repair by regulating cell proliferation, migration, chemotaxis, differentiation, morphogenesis and angiogenesis (Ornitz et al. (2001) Genome Biol 2:3005.1; Auguste et al. (2003) Cell Tissue Res 314:157; Steiling et al. (2003) Curr Opin Biotechnol 14:533). Several FGFs and FGFRs are associated with the pathogenesis of breast, prostate, cervical, stomach, and colon cancers (Jeffers et al. (2002) Expert Opin Ther Targets 6:469; Mattila et al. (2001) Oncogene 20:2791-2; Ruohola et al. (2001) Cancer Res 61:4229; Marsh et al. (1999) Oncogene 18:1053; Shimokawa et al. (2003) Cancer Res 63:6116; Jang (2001) Cancer Res 61:3541; Cappellen (1999) Nat Genet 23:18; Gowardhan (2005) Br J Cancer 92:320).
In addition, FGFR4 expression is widely distributed and was reported in tissues including developing skeletal muscles, liver, lung, pancreas, adrenal, kidney, and brain (Kan et al. (1999) J Biol Chem 274:15947; Nicholes et al. (2002) Am J Pathol 160:2295; Ozawa et al. (1996) Brain Res Mol Brain Res 41:279; Stark et al. (1991) Development 113:641). FGFR4 amplification was reported in mammary and ovarian adenocarcinomas (Jaakkola et al. (1993) Int J Cancer 54:378). Moreover, mutations and truncations of FGFR4 have correlated with malignancy, and in some cases, the prognosis of prostate and lung adenocarcinomas, head and neck squamous cell carcinoma, soft tissue sarcoma, astrocytoma and pituitary adenomas (Jaakkola et al. (1993) Int J Cancer 54:378; Morimoto (2003) Cancer 98:2245; Qian (2004) J Clin Endocrinol Metab 89:1904; Spinola et al. (2005) J Clin Oncol 23:7307; Streit et al. (2004) Int J Cancer 111:213; Wang (1994) Mol Cell Biol 14:181; Yamada (2002) Neurol Res 24:244). In addition, a polymorphism at amino acid 388 of the polypeptide sequence of FGFR4 is associated with a more aggressive disease status in melanoma (Streit al al., 2006), breast (Bange et al, 2002), prostate (Wang et al. (2004) Clin Cancer Res. 10:6169), head and neck squamous cell carcinomas (HNSCC) (Streit et al. (2004) Int J Cancer 111:213), lung adenocarcinoma (Spinola et al. (2005) J Clin Oncol 23:7307) and soft tissue sarcoma (Morimoto et al. (2003) Cancer 98:2245).
Interestingly, transgenic expression of an FGFR4 specific ligand, FGF19, under control of a muscle-specific promoter in mice has been found to lead to hepatocellular carcinoma (Nicholes et al. (2002) Am J Pathol 160:2295).
Accordingly, agents that specifically target FGFR4 and/or interfere with FGFR4-mediated signaling are desirable. Anti-FGFR4 antibodies with unique genetic and amino acid structures, including unique binding and functional characteristics, are particularly useful. Such antibodies may serve as diagnostic and therapeutic tools for a variety of diseases, including cancer.